I believe there was also an article on this kind of gadget
in Electronics Today International, (circa 1987?).
It caught my eye because I lived in a shared house,
with shared phone, at the time.

Not all residents had the patience, or honesty, to write
down their call details, so I hoped to catch the culprit out.

The ETI project had an 8751 and fitted within a standard
phone socket box. It stored data in some small non-volatile
memory which you could then dump to a centronics printer port.

You could make it prevent people from dialling high-cost
0891 phone-wank numbers, and other features.

As usual, you had to buy the expensive 8751 pre-programmed from
the designer, with no source code to check or modify.

It was some time ago, so I'm not sure if it worked with the
now ubiquitous DTMF phones.

If you do decide to do this project, consider getting it
to accept unique keys. It's all very well knowing which
numbers have been dialled, but nowadays you'll know that
eventually if you get an itemised bill. At the time, BT
didn't do this service.

What you really need to know is which one or more of your
housemates is responsible, and thus whose testicles to
remove.

I'd suggest you make the keys from Dallas Semi's "touch-id"
thingies. I've seen these used for tills in bars, where
they also have operator integrity problems.

I also suggest you make it with an infra-red RS232 link.
This is cheaper in I/O pins than a centronics printer port,
lower power, and opto-isolated as well.
I'd prefer to upload call logs than collect paper printouts.

Philips do some DTMF decoder chips.
I think one was used in Elektor's DTMF decoder project.

Keep me posted on your project.
What is its goals?